Modern elevators usually have a machinery, which drives the elevator car under control of a control unit. The control unit is typically a centralized set of electrical components but may also be in a decentralized form such that the functions are performed with distantly positioned electrical components. The machinery typically comprises a motor connected in a force transmitting manner with a traction sheave engaging an elevator roping connected to the car. Thus, the driving force can be transmitted first from the motor to the drive sheave, and from the drive sheave to the elevator car via said roping. The elevator control unit typically controls the speed of the elevator car by controlling the rotational speed of the motor of the machinery according to speed settings stored in a memory of the control unit. The speed settings define the target speed for the elevator car. The control unit controls the speed of the car so that it follows the target speed as far as possible. The speed settings typically define a constant target speed for the ongoing elevator run, i.e. a speed which stays unchanged a substantial period of time. This constant target speed is usually the maximum speed the car reaches during its run, and its length and value is typically maximized so as to provided quick transportation from a starting landing to a destination landing. In a normal run, a cycle comprising an acceleration phase from standstill, a deceleration phase to standstill, and a constant speed phase occurring between the acceleration phase and the deceleration phase is carried out. In the acceleration phase the elevator car is first accelerated from standstill to the constant target speed and in the constant speed phase the car speed is maintained constant as far as possible until the car needs to decelerate so as to smoothly arrive at the destination landing.
The speed of the elevator car needs to be prevented from exceeding a certain safety limit. Thus, in threatening situations the car can be stopped before the speed thereof increases further to a hazardous scale. Such problems may arise for example if the roping slips or the roping is cut. A safety limit of this kind is overseen by an arrangement for preventing overspeed of the car. This arrangement may be in the form of a device called overspeed governor, for instance. The elevator car is typically brought to an immediate stop if the safety limit is exceeded. This kind of safety limit is normally used to trigger an immediate, but controlled emergency stopping sequence. Additionally, a safety gear braking is triggered if the speed increases despite the emergency stopping sequence. The problem with the known elevators has been that some abnormal conditions may cause the elevator car to oscillate vertically such that at the moment of the peak of the oscillation one of these safety limits is momentarily exceeded. This problem is most likely to occur during the constant speed phase when the car speed is high. Such abnormal situations has been now noticed to include at least an earthquake, sway of the ship in case the elevator is installed in a ship, people jumping inside the car, momentarily increased resistance in sliding against the guide rails and irregular feed-back from a broken motor speed regulation sensor. These kind of abnormal situations have been noticed to cause unscheduled stops for the car, in particular due to triggering of said emergency stopping sequence or in the worst case even a safety gear braking. The unscheduled stops have the disadvantage that they decrease the efficiency and reliability of the system as well as cause inconvenience for the passengers at least for the reason that the destination landing is not reached.